1. Field of the Invention
The present invention relates to a ringing choke converter type switching power supply unit.
2. Description of the Related Art
Up to now, a ringing choke converter (hereinafter, referred to as RCC) type switching power supply unit has been known for use as a switching power supply unit. This RCC type switching power supply unit is to provide a stabilized DC power supply using a commercially available AC power supply, and, as the construction is relatively simple and the power supply unit is small-sized, light-weight, and highly efficient, it is widely used in various devices such as electronic computers, communication devices, office automation equipment, etc. Furthermore, this type of power supply unit is also utilized as an auxiliary unit and a pre-regulator in small capacity series regulators.
FIG. 8 is a circuit diagram of a conventional RCC type switching power supply unit. As shown in the drawing, the switching power supply unit 10 is composed of an input circuit 2, a DC-DC converter circuit 3, a voltage detector circuit 4, and a control circuit 5.
The input circuit 2 is composed of a fuse F, a filter circuit LF, and a rectifying diode bridge DB, and an alternating-current power supply AC is connected to the input terminals. Furthermore, one input terminal of the input circuit 2 is connected to one end of the input-side of the filter circuit LF through the fuse F and the other input terminal of the input circuit 2 is directly connected to the other end of the input side of the filter circuit LF. Furthermore, the terminals on the output side of the filter circuit LF are connected to respective input terminals of the diode bridge DB. The output terminals a and b of the diode bridge DB are connected to the input terminals a and b of the DCxe2x80x94DC converter circuit 3.
The DCxe2x80x94DC converter circuit 3 is composed of capacitors C1 and C4, a transformer T, a main switching element such as MOS field-effect transistor (FET) Q1 as resistors R1 and R10, and a diode D1. Furthermore, the transformer T contains a primary winding N1, a secondary winding N2 which is opposite in polarity to the primary winding N1, and a feedback winding Nb which is the same in polarity as the primary winding N1. The smoothing capacitor C1 is provided between the input terminals a and b of the DCxe2x80x94DC converter circuit 3, and also a series circuit of the primary winding N1 and the FET Q1 is provided so as to be parallel to the capacitor C1. Thus, the starting point of the primary winding N1 of the transformer T is connected to the input terminal a of the DCxe2x80x94DC converter circuit 3, the drain of the FET Q1 is connected to the end of the primary winding N1, and the source of the FET Q1 is connected to the input terminal b of the DCxe2x80x94DC converter circuit 3. Furthermore, one end of the starter resister R1 is connected to the starting point of the primary winding N1, and the other end of the resistor R1 is connected to the gate of the FET Q1. The resistor R10 is connected between the gate and the source of the FET Q1. Moreover, the anode of the rectifying diode D1 is connected to the end of the secondary winding N2 of the transformer T, and the smoothing capacitor C4 is connected between the cathode of the diode D1 and the starting point of the secondary winding N2. The diode D1 and capacitor C4 constitute a rectifying and smoothing circuit.
The voltage detector circuit 4 is provided on the output side of the DCxe2x80x94DC converter circuit 3, and is composed of resistors R5, R6, and R7, a light-emitting diode PD as a light-emitting device of a photo coupler PC, and a shunt regulator Sr. The output terminals of the voltage detector circuit 4 are provided so as to be parallel to the capacitor C4 of the DCxe2x80x94DC converter circuit 3. Furthermore, each of a series circuit of the resistor R5, the light-emitting diode PD, and the shunt regulator Sr, and a series circuit of the resistors R6 and R7 is provided so as to be parallel to the capacitor C4. Moreover, the cathode of the light-emitting diode PD and the cathode of the shunt regulator Sr are connected. Furthermore, the reference terminal of the shunt regulator Sr is connected to the connection point between the resistors R6 and R7.
The control circuit 5 is composed of resistors R2, R3, R4, and R13, capacitors C2 and C3, a diode D2, a phototransistor PT as a light receiving element of a photo coupler PC, and an NPN transistor Q2. The capacitor C3 and the resistor R13 are connected in series between the gate of the FET Q1 of the DCxe2x80x94DC converter circuit 3 and the starting point of the feedback winding Nb. Furthermore, the collector and the emitter of the transistor Q2 are connected to the gate and the source of the FET Q1, respectively. Moreover, a series circuit of the resistor R2 and the resistor R3 is connected between the starting point and the end of the feedback winding Nb. In addition, a series circuit of the resistor R4, the diode D2, and the phototransistor PT in the photo coupler PC is connected so as to be parallel to the resistor R2. Moreover, the cathode of the diode D2 is connected to the collector of the phototransistor PT in the photo coupler PC. Furthermore, the capacitor C2 is connected between the base and the emitter of the transistor Q2, and the above resistor R3 is connected so as to be parallel to the capacitor C2.
Next, the operation of the switching power supply unit 10 constructed as in the above is described. First, at the start where the power supply is turned on, a voltage is applied to the gate of the FET Q1 through the starter resistor R1 and the FET Q1 is turned on. In this way, the voltage of the power supply is applied to the primary winding N1 of the transformer T and a voltage having the same polarity as that of the primary winding N1 is generated in the feedback winding Nb. This voltage signal as a positive feedback signal is provided to the gate of the FET Q1 through the resistor R13 and the capacitor C3, and the FET Q1 is quickly turned on. At this time, an excitation energy is stored in the primary winding N1.
A charging current flows into the capacitor C2 through the resistor R2 because of the electromotive force in the feedback winding Nb. When the charging voltage of the capacitor C2 exceeds the forward voltage drop between the base and the emitter of the controlling transistor Q2, the transistor Q2 is turned on. In this way, the voltage between the gate and the source of the FET Q1 becomes substantially zero thereby to turn off the FET Q1.
Thus, the excitation energy stored in the primary winding N1 of the transformer T during the ON-state period of the FET Q1 is discharged as electric energy through the secondary winding N2, rectified by the diode D1, smoothed by the capacitor C4, and supplied to the load (not shown in the drawing) which is connected to the output terminals of the voltage detector circuit 4.
When the electric charge in the charged capacitor C2 is discharged through a discharge circuit of the resistor R3, etc., and the charged voltage becomes equal to or less than the forward voltage drop between the base and the emitter of the transistor Q2, the transistor Q2 is turned off. Furthermore, when the excitation energy stored in the primary winding N1 of the transformer T is discharged through the secondary winding N2 and the current flowing through the diode D1 becomes zero, a kickback voltage is generated in the feedback winding Nb in which the starting point of the winding is positive polarity and the FET Q1 is turned on by the kickback voltage once again. When the FET Q1 is turned on, a voltage is provided to the primary winding N1 of the transformer T again, and an excitation energy is stored in the primary winding N1.
Thus, the above-described oscillation is repeated in the switching power supply unit 10.
Next, the feedback control by the voltage detector circuit 4 is described. Under normal conditions, the output voltage on the load side is detected by a voltage divider of the resistors R6 and R7, and the detected voltage and the reference voltage of the shunt regulator Sr are compared. The changes of the output voltage are amplified by the shunt regulator Sr, a current passing through the light-emitting diode PD of the photo coupler PC changes, and the amount of light emission of the light-emitting diode PD also changes. Furthermore, in accordance with the amount of light received by the phototransistor PT of the photo coupler PC, the impedance of the phototransistor PT changes and accordingly the charging time constant of the capacitor C2 changes. The more the output voltage decreases, the larger the above charging time constant becomes, and accordingly the more the output voltage decreases, the longer the period from the turn on to the forcible turn off, caused by the transistor Q2, of the FET Q1, that is, the ON-state period of the FET Q1, increases, forcing the output voltage to increase. Furthermore, when the output voltage increases, the condition is reversed and the output voltage is forcibly reduced. In this way, a constant-voltage control is achieved so that the output voltage may be constant.
Moreover, the conditions of a light load correspond to the cases where the above-mentioned output voltage increases, and the conditions of a heavy load correspond to the cases where the above output voltage decreases.
In the conventional RCC type switching power supply unit shown in FIG. 8, it is known that the oscillation frequency of the FET Q1 changes substantially inversely with the input power or output power. This is shown by (b) in FIG. 5, that is, shown as the relationship of the oscillation frequency (switching frequency) to the output (load power).
Generally, as the load becomes lighter, the switching loss is reduced for each switching operation, but, as shown by (b) in FIG. 5, the smaller the output power is, that is, the lighter the load is, the higher the oscillation frequency becomes, and then the higher the oscillation frequency becomes, the greater the number of switching operations and accordingly, switching losses increase. As a result, even if the load becomes lighter, the degree of reduction of the switching losses is very little. Therefore, the lighter the load is, the more the efficiency of the power supply unit is reduced.
In order to reduce the switching loss under such light-load conditions, the circuit constants may be designed so that the oscillation frequency becomes lower at the rated load, but when the power supply unit is required to cope with a wide range of very light loads to heavy loads, it is unavoidable that the oscillation frequency becomes relatively higher at a light load. That is, generally the oscillation frequency at the rated load is mainly determined by effects in the parts such as the magnetic flux density of the transformer and of ripples, noise, etc., and if the oscillation frequency is made too low, there is a problem in that the saturation of the transformer, etc., are caused.
Furthermore, when the device using the conventional RCC type switching power supply unit is on standby, that is, at the above-mentioned light load, as the oscillation frequency becomes higher, heat generation is caused in the main switching element because of the switching losses.
For example, in the switching power supply unit used in a printer, when the power switch is in the OFF-state, it is possible to operate the power source in an optimized operation mode during standby by using a switching signal. On the other hand, once the power switch of the printer is turned on for printing, etc., the switching power supply unit returns to the optimized operation mode for normal work. Furthermore, when the power switch of the printer is in the ON-state, if printing is not performed, there is a waiting time. Because the printer is light-loaded during this waiting time, heat is generated in the main switching element of the switching power supply unit used in the printer as described above.
Regarding this problem, in Japanese Unexamined Patent Application Publication No. 11-235036, the inventors of the application concerned have disclosed a technology concerning a switching power supply unit in which the loss on standby is improved by inputting a switching signal on standby and reducing the oscillation frequency.
Furthermore, in Japanese Patent Application No. 11-253550, the inventors of the application concerned have proposed a technology concerning a switching power supply unit in which the loss on standby is improved by continuously lowering the oscillation frequencies at the rated range of use on standby.
However, in the switching power supply unit disclosed in Japanese Unexamined Patent Application Publication No. 11-235036, when the switching signal is for normal operation, the power supply unit functions as an RCC. Therefore, when the power supply unit is on standby as in the above-described printer, no means for reducing the loss is provided and there is a problem in that the increase of the input power and heat generation in the main switching element are caused. Furthermore, there is also an operation of an intermittent oscillation and in this case there is a problem in that the output ripple voltage increases.
Furthermore, in the switching power supply unit proposed in Japanese Patent Application No. 11-253550, when the power supply unit is lightly-loaded, the oscillation frequency is automatically reduced. In this case, when the oscillation frequency is reduced too much, the response characteristic of the load worsens, and accordingly the frequency is limited to high frequencies when compared to the technology described in Japanese Unexamined Patent Application Publication No. 11-235036. Because of this, there is a problem in that the effect of lowering the loss is inferior to a switching signal input type.
The present invention is to solve the above-mentioned problems, and it is an object of the present invention to provide an RCC type switching power supply unit in which by lowering or keeping the oscillation frequency substantially at a constant value at a light load and on standby and thus avoiding the increase of the oscillation frequency, the suppression of the standby power, the suppression of heat generation of the main switch, the standby efficiency of the RCC, and the improvement of the output ripple voltage caused by the intermittent oscillation are objectives, and in which by switching the operation mode by an external signal or switching the operation mode after a certain period of time after the load current has become on standby, the oscillation frequency is further lowered and the loss on standby is greatly reduced.
The ringing choke converter type switching power supply unit of the present invention comprises a transformer having a primary winding, a secondary winding, and a feedback winding; a main switching element receiving a feedback signal from the feedback winding and turning on and off the current of the primary winding; a rectifying and smoothing circuit comprising a rectifying element and a smoothing element, being connected to the secondary winding; and a control circuit connected between the feedback winding and the control terminal of the main switching element, a delay circuit for prohibiting the turning on of the main switching element for a certain period of time after the current flowing through the rectifying element has become zero is provided, and a switching circuit for switching the delay time of the delay circuit in two stages or more than two stages is provided.
According to this construction, the turning on of the main switching element is prohibited for a certain period of time by the delay circuit and the OFF-state period is extended. Furthermore, the delay time can be switched in two stages or more than two stages by the switching circuit. Therefore, the switching frequency of the main switching element is lowered when compared to the cases where such a delay circuit is not used. Furthermore, it becomes possible to set the switching frequency of the main switching element at a desired value by the switching circuit.
Furthermore, in the switching power supply unit of the present invention, the delay circuit contains a switching device and a time constant circuit which are provided between the feedback winding of the transformer and the control terminal of the main switching element is provided, and a switching circuit for switching the time constant of the time constant circuit is provided. In this way, the ON-state period of the switching device changes in accordance with the time constant of the time constant circuit, and it becomes possible to switch the time constant of the time constant circuit by the switching circuit.
Moreover, in the switching power supply unit of the present invention, an output power detector circuit for rectifying and smoothing the voltage of the feedback winding of the transformer by a rectifying element and a smoothing element is provided, and a delay time extension circuit connected between the output power detector circuit and the control terminal of the delay circuit through a resistor element is provided.
In addition, in the switching power supply unit of the present invention, the switching circuit can be switched by an external signal. In this way, it becomes possible to externally switch the switching frequency of the main switching element easily.
Furthermore, in the switching power supply unit of the present invention, a current detector circuit for detecting the current flowing through the secondary winding is provided, and the switching circuit for switching the delay time of the delay circuit after a certain period of time when the current detector circuit detects the load current or load power being on standby is provided. Thus, it becomes possible to switch the switching frequency of the main switching element automatically.